The present invention relates to an ingestible device. In particular the invention relates to such a device in the form of a capsule that is intended to release a controlled quantity of a substance, such as a pharmaceutically active compound, foodstuff, dye, radiolabelled marker, vaccine, physiological marker or diagnostic agent at a chosen location in the gastrointestinal (GI) tract of a mammal. Such a capsule is sometimes referred to as a “Site-Specific Delivery Capsule”, or SSDC.
SSDC's have numerous uses. One use of particular interest to the pharmaceutical industry involves assessing the absorption rate and/or efficacy of a compound under investigation, at various locations in the GI tract. Pharmaceutical companies can use data obtained from such investigations, eg. to improve commercially produced products.
Several designs of SSDC are known. One design of capsule intended for use in the GI tract of a mammal is disclosed in “Autonomous Telemetric Capsule to Explore the Small Bowel”, Lambert et al, Medical & Biological Engineering and Computing, March 1991. The capsule shown therein exhibits several features usually found in such devices, namely:                a reservoir for a substance to be discharged into the GI tract;        an on-board energy source;        a mechanism, operable under power from the energy source, for initiating discharge of the substance from the reservoir;        a switch, operable remotely from outside the body of the mammal, for initiating the discharge; and        a telemetry device for transmitting data indicative of the status, location and/or orientation of the capsule.        
Also, of course, the dimensions of the capsule are such as to permit its ingestion via the oesophagus; and the external components of the capsule are such as to be biocompatible for the residence time of the capsule within the body.
The capsule disclosed by Lambert et al suffers several disadvantages. Principal amongst these is the complexity of the device. This means that the capsule is expensive to manufacture. Also the complexity means that the capsule is prone to malfunction.
For example, the capsule disclosed by Lambert et al includes a telemetry device that is initially retracted within a smooth outer housing, to permit swallowing of the capsule via the oesophagus. Once the capsule reaches the stomach, gastric juice destroys a gelatin seal retaining the telemetry device within the housing. The telemetry device then extends from the housing and presents a rotatable star wheel that engages the wall of the GI tract. Rotations of the star wheel generate signals that are transmitted externally of the capsule by means of an on-board RF transmitter powered by a battery within the capsule housing.
This arrangement may become unreliable when used in mammals whose GI motility is poor or whose gastric juice composition is abnormal.
There is a risk of malfunction of the rotating part of the telemetry device, and the method of operation of the capsule is generally complex.
The space needed to house the telemetry device within the capsule during swallowing/ingestion is unusable for any other purpose when the telemetry device is extended. Therefore the Lambert et al capsule is not space-efficient. This is a serious drawback when considering the requirement for the capsule to be as small as possible to aid ingestion.
Also the Lambert et al disclosure details the use of a high frequency (>100 MHz) radio transmitter for remotely triggering the release of the substance from the capsule into the GI tract. The use of such high frequencies is associated with disadvantages, as follows:                When power is transmitted to the capsule whilst it is inside the GI tract the energy must pass through the tissue of the mammal that has swallowed the capsule. The transmission of this power through the body of the mammal may result in possible interactions with the tissue which at some power levels may lead to potential damage to that tissue.        The higher the frequency of energy transmission the higher the coupled power for a given field strength. However, as the frequency is increased the absorption of the energy by the body tissue also increases. The guidelines for the exposure of humans to static and time varying electromagnetic fields and radiation for the UK are given in the National Radiological Protection Board (NRPB) publication “Occupational Exposure to Electromagnetic fields: Practical Application of NRPB Guidance” NRPB-R301. This describes two mechanisms of interaction: induced currents and direct heating measured in terms of the SAR (specific energy absorption rate). In general terms the induced current dominates up to 2 MHz above which the SAR effects take over.        